The present invention relates generally to ink jet printers and, more particularly, to an apparatus for sensing discrete liquid levels in an ink jet printing system and for maintaining satisfactory operation thereof.
In one type of ink jet printing system, a supply of an electrically conductive ink under pressure is subjected to vibratory energy to create a jet of regularly spaced drops. The drops are selectively charged by charging electrodes and pass through an electrostatic deflection field created by a pair of high voltage plates. The drops follow differing trajectories depending on the level to which the drops are individually charged. Certain drops are deposited on the substrate to form the image, while the remaining drops are intercepted by a catcher to be recirculated. See, for example, U.S. Pat. No. 4,555,712 and 4,827,280. An ink reservoir is included in such systems to collect ink deposited in the catcher for reuse.
Reservoirs 54 or 85 in the '712 patent and reservoir 12 in the '280 patent each serve as a holding tank for ink which is transferred to the pressure cylinder to be delivered under pressure to the nozzle. Ink which is collected by the catcher typically loses solvent content that is replenished periodically by adding makeup fluid into the reservoir. Also, fresh ink is added periodically to replace the ink consumed during printing. The reservoir serves as a mixing chamber for all of these fluids such that the level of fluid in the reservoir constantly changes.
While ink is transferred to the pressure cylinder and makeup fluid and fresh ink are added, the fluid level in the reservoir can be sensed at several specific points, including (i) a lowest fluid or ink level, below which makeup fluid should be added to avoid exhausting the supply of ink; (ii) an intermediate fluid level, identified as the "reservoir full" state; and (iii) an upper fluid level, identified as an "overfull" state which causes immediate shut-down of the printing system.
One method of sensing fluid levels is to use a float containing a magnet. The float rises and falls with the liquid level and its relative position is detected at specific heights by magnetic sensors such as magnetic reed switches or other magnetically activated devices. Magnetic floats and reed switches have commonly been used for many years in the art of ink jet printing.
There are reliability problems with such float based sensors in the reservoir. Accurate liquid level sensing from system to system can be difficult to achieve in the manufacturing process due to the variation in magnet strength, switch sensitivity and variations in the positional relationship between the two varying as the float moves over extended use. There is an uncertainty in knowing the fluid level whenever the float is between switches. Dried ink in a printer which has been inoperative for an extended period of time can cause a float to stick in a fixed position.
The problems with floats can be avoided by utilizing a plurality of electrically conductive rods to ascertain the level of the conductive liquid ink in a reservoir or container. However, the use of such rods is affected by ink froth, foam and mist in the ink reservoir or container. Because the froth, foam and mist are electrically conductive, the level sensing rods can be shorted together, thereby allowing erroneous liquid level measurements to be made.
What is desired is an improved liquid level sensor capable of providing accurate measurements of liquid levels in ink jet printing systems. More specifically, an improved ink level sensor which avoids the inaccuracies resulting from frothing, spattering, foaming and misting is desired.